1. Field of the Invention
My invention relates generally to cardiac stimulation, and more particularly to an implantable endocardial lead which stimulates or senses electrical activity of the heart and which employs a spring-loaded fixation mechanism comprising a sharpened helix which screws itself into cardiac tissue without rotation of the lead or a stylet.
2. Prior Art
There are generally two types of body implantable leads used with cardiac pacemarkers--one which requires surgery to expose the myocardial tissue to which an electrode is affixed and another which can be inserted through a body vessel, such as a vein, into the heart where an electrode contacts the endocardial tissue. In the latter type, the endocardial lead is often secured to the heart through the endothelial lining by a sharpened helix affixed to a distal end of the lead. When the end of the lead contacts the lining of the heart at a desired location, the lead may be secured in place by rotating the lead, thus screwing the helix into the heart tissue.
A helix system has been relatively effective in secureing an endocardial lead once the initial location of the lead has been achieved. However, it is undesirable to expose the sharpened helix while the lead is being inserted through a blood vessel into the heart. Moreover, to implant the helix into the heart, it has generally been proposed that the helix be rotated either by rotating the entire lead or by rotating a stylet or other structure within the lead.
For example, the following patents illustrated leads wherein the helix is rotated by the rotation of the entire lead: U.S. Pat. No. 3,974,834 to Kane; U.S. Pat. No. 4,570,642 to Kane, and Revane; U.S. Pat. No. 4,649,938 to McArthur; U.S. Pat. No. 4,924,881 to Brewer and U.S. Pat. No. 4,967,716 to Bradshaw.
U.S. Pat. No. 4,463,765 to Gold for a "Screw-in Pacing Lead Assembly", on the other hand, illustrates the use of an internal structure to rotate the helix. A flat ribbon extending throughout the lead from a proximal to a distal end thereof is disclosed. Another internal apparatus is disclosed in U.S. Pat. No. 5,056,516 to Paul R. Spehr. An endocardial lead with a flexible, tubular landyard is described. The lanyard passes through a lumen from a proximal end of the lead to a distal end of the lead where the lanyard was attached to a sliding member supporting a helix. When the helix is in an exposed position, torque can be transmitted from the proximal end of the lanyard from the distal end thereof through the piston and then to the helix to screw the helix into the endocardial tissue.
Other apparatus for securing an electrode within the heart are also known. For example, U.S. Pat. No. 3,754,555 to Schmitt discloses a lead having prongs which can be thrust out from and retracted into an electrode, U.S. Pat. No. 4,858,623 to Bradshaw and Baker discloses a lead having a pivoting rigid hook. In a first position, the hook is collapsed against the electrode and not exposed to tissue. In a second position, the hook is extended and can engage a cardiac wall.